This invention relates generally to a method for acoustically transmitting data along a drillstring, and more particularly to a method of enhancing acoustic data transmissions by identifying the frequency stop-bands of the drillstring locally, within the section housing the acoustic transmitter.
Deep wells of the type commonly used for petroleum or geothermal exploration are typically less than 30 cm (12 inches) in diameter and on the order of 2 km (1.5 miles) long. These wells are drilled using drillstrings assembled from relatively light sections (either 30 or 45 feet long) of drill pipe that are connected end-to-end by tool joints, additional sections being added to the uphole end as the hole deepens. The downhole end of the drillstring typically includes a drill collar, a dead weight assembled from sections of relatively heavy lengths of uniform diameter collar pipe having an overall length on the order of 300 meters (1000 feet). A drill bit is attached to the downhole end of the drill collar, the weight of the collar causing the bit to bite into the earth as the drillstring is rotated from the surface. Sometimes, downhole mud motors or turbines are used to turn the bit. Drilling mud or air is pumped from the surface to the drill bit through an axial hole in the drillstring. This fluid removes the cuttings from the hole, provides hydrostatic head which controls the formation gases, and sometimes provides cooling for the bit.
Communication between downhole sensors of parameters such as pressure or temperature and the surface has long been desirable. Various methods that have been tried for this communication include electromagnetic radiation through the ground formation, electrical transmission through an insulated conductor, pressure pulse propagation through the drilling mud, and acoustic wave propagation through the metal drillstring. Each of these methods has disadvantages associated with signal attenuation, ambient noise, high temperatures, and compatibility with standard drilling procedures. The most commercially successful of these methods has been the transmission of information by pressure pulse in the drilling mud (known as mud pulse telemetry). However, attenuation mechanisms in the mud limit the transmission rate to less than 3 bit per second.
Faster data transmission may be obtained by the use of acoustic wave propagation through the drillstring. While this method of data transmission has heretofore been regarded as impractical, a significantly improved method and apparatus for the acoustic transmission of data through a drillstring is disclosed in U.S. patent application Ser. No. 605,255 filed Oct. 29, 1990, which is a continuation-in-part of U.S. application Ser. No. 453,371 filed Dec. 22, 1989 (all of the contents of which are fully incorporated herein by reference) which will permit large scale commercial use of acoustic telemetry in the drilling of deep wells for petroleum and geothermal exploration.
U.S. Ser. No. 605,255 describes an acoustic transmission system which employs a transmitter for converting an electrical input signal into acoustic energy within the drill collar. The transmitter includes a pair of spaced transducers which are controlled by a digital circuit. This digital circuit controls phasing of electrical signals to and from the transducers so as to produce an acoustical signal which travels in only one direction.
In acoustic data transmissions along a segmented tubular structure such as a drill pipe used for drilling a well as described above, there exists both passband and stop-band frequency domains. The frequencies of these bands are determined by the material and dimensions of the tubular structure as well as the geometry of the segments. Data can be transmitted readily at the passband frequencies, but signals at the stop-band frequencies are rapidly attenuated and thus lost. Also, within the passbands there is a fine structure of low loss passbands interspersed with bands where very high attenuation occurs. These fine structure bands are described in some detail in an article entitled "Acoustical Properties of Drillstrings" by Douglas S. Drumheller, J. Acoust. Soc. Am 85 (3), pp. 1048-1064, March 1989. As described in the Drumheller paper, the fine structure bands are caused by the destructive interference of acoustic waves reflected from the ends of the tube with the original signal wave, when the two waves arrive at the receiver substantially out of phase. As a result of this fine structure phenomenon, the passband frequencies depend upon the overall length of the tube. This makes for difficulties in transmitting data when the overall length of the tube is changing, as in drilling operations where the depth of the well, and hence the length of the tube (drill pipe) is constantly increasing thereby shifting the fine structure bands. Because of the presence of this fine structure and the constantly changing nature of the fine structure, it is very difficult to determine the optimal transmission frequency and thereby accurately transmit acoustic data signals.